Powering Your Data Story with Sankey Charts: A Visual Guide to Flow, Cause, and Effect

In the realm of data visualization, few tools are as powerful and compelling as the Sankey chart. Named after Mark Sankey, a civil engineer who developed these charts in the late 19th century for assessing the efficiency of steam engines, today they have become an indispensable tool for understanding complex flows and illustrating cause-and-effect relationships. Whether you’re analyzing energy use, mapping the flow of information, examining the path of pollutants through a water system, or exploring consumer purchasing habits, Sankey charts provide a clear, engaging way to convey data in a way that’s easy to understand and compelling to look at.

Understanding Sankey Charts

Sankey charts, also known as flow diagrams, consist of a network of parallel bars, also known as links, which widths vary according to the magnitude of the data they represent. Each bar represents a flow from one process to another, similar to the way a map shows the routes of rivers. These charts are particularly useful for displaying data that involves a flow or distribution in both quantity and direction, making them ideal for tracking the progression, depletion, or transformation of resources or information across systems.

Creating Sankey Charts

Creating a Sankey chart involves several steps, from data preparation to charting and final adjustments. Here’s a simplified guide:

Data Preparation: Gather your data, ensuring that you have a comprehensive set that includes both the source and destination points of the flow, along with the quantity or volume of the data. This data will be crucial in determining the width and relative proportions of the links in your Sankey chart.
Sorting and Normalization: Sort your data to highlight the most significant flows and normalize the data to ensure that the links in your chart are proportional and visually meaningful. This process is essential for making the chart readable and easy to interpret.
Using Software Tools: For those unfamiliar with code, there are numerous software tools and apps, both free and commercial, that can help create Sankey charts. Popular options include Python libraries like ‘Plotly’ or ‘Bokeh’, Excel macros, R-packages, and web-based platforms like Tableau or D3.js.
Creating the Chart: Once you have your data organized and your software ready, it’s time to begin creating the chart. This typically involves setting up the initial nodes (source and destination points), calculating the widths of the links based on your data, and configuring the colors and text to enhance readability and engagement.
Final Adjustments: Review your chart for clarity and effectiveness. Adjust the size, color, and positioning of elements to ensure that the information is presented in the most logical, compelling way. This could involve adding labels for nodes or links, changing the chart orientation, or applying gradients for a more visually appealing presentation.

Applications of Sankey Charts

The versatility of Sankey charts makes them suitable for a wide range of applications across various fields:

Energy Analysis: Sankey diagrams are commonly used to visualize the efficiency of energy systems, showing the flow of energy from generation to use.
Marketing and Consumer Behavior: Brands use them to understand how products flow through different channels or how consumers’ purchasing behaviors change over time.
Network Traffic Analysis: Professionals can use Sankey diagrams to visualize data flow in networks, identifying bottlenecks or inefficiencies.
Environmental Science and Sustainability: These charts are essential tools for assessing how pollutants or waste products move through ecosystems and human systems.

Conclusion

Sankey charts are a powerful and visually engaging way to tell stories with data, making complex flows and cause-and-effect relationships accessible and understandable. By following the steps to create these charts and understanding their applications, you can transform raw data into compelling visual stories that communicate insights and inform decision-making. The key to success with Sankey charts lies in understanding your audience and the message you want to convey, ensuring that your visualization supports and enhances that narrative.

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